Field
The present disclosure relates to an organic light emitting device and more particularly, to an organic light emitting device having a multi-layer light emitting structure and capable of improving a color change rate based on viewing angle, efficiency, and a lifetime of the device.
Description of the Related Art
An organic light emitting display (OLED) device is a self-light emitting display device and does not need a separate light source as in a liquid crystal display device. Thus, the OLED device can be manufactured into a lightweight and thin form. Further, the OLED device is advantageous in terms of power consumption since it is driven with a low voltage. Also, the OLED device has excellent color expression ability, a high response speed, a wide viewing angle, and a high contrast ratio (CR). Therefore, the OLED device is being developed as a next-generation display device.
The OLED device includes an organic emitting layer that emits light by a reactive combination of electrons and holes. Generally, an organic light emitting device includes an anode, a hole injection layer (HIL), a hole transport layer (HTL), an organic emitting layer (EML), an electron transport layer (ETL), an electron injection layer (EIL), and a cathode.
Recently, an OLED device having a tandem structure (i.e. a stack of two or more layers or two or more multi-layer light emitting units) manufactured by laminating a plurality of organic emitting layers has been actively researched to improve luminous efficiency and a lifetime of an organic light emitting device. Specifically, each light emitting unit includes an organic EML and may further include an HIL, an HTL, an ETL, and an EIL.
If a plurality of light emitting units is laminated as in the OLED device having a tandem structure, it is difficult for holes and electrons generated from an anode and a cathode to be smoothly transferred to each of the plurality of organic light emitting layers. Thus, efficiency of the organic light emitting device may be reduced. Therefore, in order to improve the efficiency of the organic light emitting device, a charge generation layer capable of efficiently supplying electrons and holes to the organic emitting layers is disposed between the plurality of light emitting units.
Hereinafter, a conventional organic light emitting device having a tandem structure including two light emitting units will be described.
The conventional organic light emitting device having a tandem structure including two light emitting units has a structure in which an anode, a first light emitting unit, a charge generation layer, a second light emitting unit, and a cathode are laminated in sequence.
The first light emitting unit is disposed on the anode and configured to emit a light of a first color. Specifically, the first light emitting unit may include an HIL, a first HTL, a first organic EML that emits the light of the first color, and a first ETL.
The second light emitting unit is disposed on the charge generation layer and configured to emit a light of a second color different from the first color. Specifically, the second light emitting unit may include a second HTL, a second organic EML that emits the light of the second color, a second ETL, and an EIL.
The charge generation layer is disposed between the first light emitting unit and the second light emitting unit and configured to regulate a balance of electrons and holes to be supplied to the first light emitting unit and the second light emitting unit, respectively. Specifically, the charge generation layer may include an N-type charge generation layer that transfers electrons to the first light emitting unit and a P-type charge generation layer that transfers holes to the second light emitting unit.
The organic light emitting device having a tandem structure may emit a white light by mixing lights emitted from the first light emitting unit and the second light emitting unit.
However, in the conventional organic light emitting device having a tandem structure, the N-type charge generation layer and the P-type charge generation layer constituting the charge generation layer are formed by separate deposition processes. Further, the HTL and the ETL constituting each light emitting unit are also formed by separate deposition processes. Therefore, numerous manufacturing processes are required.
Accordingly, in the conventional organic light emitting device having a tandem structure, various layers are laminated as components and a gap between the organic EMLs is increased by the laminated layers. If the gap between the organic EMLs is increased, an electroluminescence (EL) spectrum in a front view is greatly different from an EL spectrum in a side view. Therefore, optical characteristics related to a color change rate based on a viewing angle may deteriorate.
Meanwhile, the ETL is disposed between the organic EML and the N-type charge generation layer and configured to supply electrons from the N-type charge generation layer to the organic EML. However, if the ETL does not have a high mobility in the organic light emitting device including the multi-layer light emitting unit, electrons cannot be smoothly transferred from the N-type charge generation layer to the organic EML at a low current density, i.e., from about 0.01 mA/cm2 to about 1 mA/cm2. Thus, a resistance of the organic light emitting device is increased due to the remaining holes, which are not combined with electrons, among all holes transferred from the HTL to the organic EML. Therefore, as the resistance of the organic light emitting device is increased, a driving voltage of the organic light emitting device needs to be increased and thus a lifetime of the organic light emitting device is accordingly decreased.